Dendrite or asteroidal titanium dioxide micro-particles

ABSTRACT

Dendrite or asteroidal titanium dioxide micro-particles having a length of 0.2 to 0.5 μm, a thickness of 0.05 to 0.1 μm, and a specific surface area of 20 to 130 m 2  /gram, are produced by treating hydrated titanium oxide particles with an alkali, mixing instantaneously the resulting reaction product with an amount of hydrochloric acid in a ratio of 1 to 4 mols of hydrochloric acid to one mol of the titanium oxide in the reaction product to effect a reaction, then aging under heat at 85° C. or higher, and then drying or, if necessary, firing after the drying, and subsequently may be coated with at least one from oxides and hydrated oxides of elements selected from aluminum, silicon, titanium, zirconium, tin and antimony to improve the dispersibility and resistance to light of the particles, or may be coated with tin oxide containing antimony or indium oxide containing tin to make the particles electroconductive. These titanium dioxide micro-particles are useful for use in electroconductive paints, electroconductive resin compositions, paints and electroconductive paints for magnetic recording media, sunscreen cosmetics, UV screening paints and UV shielding plastic compositions and the like.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to dendrite or asteroidal titanium dioxidemicro-particles, a process for producing the same, electroconductivedendrite or asteroidal titanium dioxide micro-particles comprising saidtitanium dioxide micro-particles as substrate which have been subjectedto an electroconductivity imparting treatment, and a process forproducing the same. The titanium dioxide micro-particles according tothe present invention are useful for sunscreen preparations orcosmetics, UV screening paints, UV screening or shielding materials inplastics, electroconductivity imparting or antistatic materials to beused in plastics, paints, rubbers and fibers, as well aselectroconductivity imparting agents or substrates to be used insupports for recording materials such as electrophotographic copyingpaper and electrostatographic recording paper.

2. Description of the Related Art

Fine titanium dioxide particles having a primary particle size of 0.1 μmor less are transparent because they transmit rays of visible light whenincorporated in resin films or shapes, and on the other hand capable ofshielding ultraviolet radiation to protect materials which may bediscolored or degenerated with ultraviolet radiation. Thus they havedifferent properties from those of pigment titanium dioxide having aprimary particle size of about 0.15 to 0.5 μm so that they have beenutilized in sunscreen preparations, UV screening paints, and UVscreening or shielding materials for plastics.

With respect to the transparency and UV shielding property of theaforementioned titanium dioxide micro-particles, the former propertyincreases inversely proportional to the particle size, while the lattervaries with the particle size with reaching a maximum in a specificrange of the particle size and then decreasing as the particle sizeincreases over the specific range. That is, the UV shielding property isreduced at particle sizes either smaller or larger than the specificrange. Therefore, development of titanium dioxide micro-particles havingsimultaneously satisfactory transparency and UV shielding property hasbeen desired.

The aforementioned titanium oxide micro-particles having a averageprimary particle size of about 0.05 to 0.1 μm have been produced by, forexample, a process comprising firing hydrated titanium dioxides producedby hydrolyzing a solution of titanyl sulfate or titanium tetrachloride,or a titania sol obtained by thermally treating the hydrated titaniumdioxides in the presence of hydrochloric acid after treated with acaustic alkali. However, sintering of particles are highly apt to occurin the course of the firing process, so that it is difficult to achievesubstantial dispersion of the primary particles in various media fornumerous applications resulting in unsatisfactory UV shielding effectwhich is vigorously desired to be improved.

Titanium oxide compounds which have been proposed as electroconductivityimparting materials include (1) spherical electroconductive titaniumoxide particles which substrates are spherical titanium oxide particlesor fine spherical titanium oxide particles, (2) electroconductivematerials comprising primarily fibrous potassium titanate, and (3)acicular electroconductive titanium oxide particles based on highquality acicular titanium oxide particles having a length of 1 to 10 μm.

Generally speaking, electrical conductivity-impartig materials of anacicular form (including fibrous form), as compared with spherical form,and furthermore the materials which are lower in their powderresistivity, namely, higher in electrical conductivity, can give resinarticles and rubber articles of the desired conductivity even with theaddition thereof in a small amount to the article. The sphericalelectroconductive titanium oxide particles referred to in the above item(1) are generally required to be incorporated in a considerably largeamount into media such as resins and rubbers to impart desiredelectroconductivities to the media due to spherical particles, thoughtheir powder bodies have a very excellent electroconductivity, forexample, a resistivity of 1 to 10 Ω cm. For this reason, the use ofspherical titanium oxide particles produces problems of reducing thestrength of the products such as electroconductive resins and rubbersand of being uneconomical. The electroconductive materials referred toin the above item (2) have no problem in configuration, butdisadvantages that the powder body of the materials has a highresistivity and the fibrous materials are liable to break duringdispersing. Acicular electroconductive titanium oxide particles referredto in the above item (3) overcome the problems described above withrespect to (1) and (2) in that only a small amount of the acicularparticles is required to be added to achieve a very excellentelectroconductivity, but they cause problems of flatness of the surfacesof coatings obtained by spreading a paint composition containing theacicular particles and a medium on a substrate, because they are toolong in addition to being acicular. There is a need for titanium oxideparticles which are useful for an antistatic agent for magneticrecording media, more excellent in transparency and surface flatness ofmagnetic layers as well as in electroconductivity imparting effectaccording to a demand for increasing recording density.

SUMMARY OF THE INVENTION

An object of the present invention is to provide titanium dioxidemicro-particles having a specific length, a specific thickness, aspecific surface area and a specific configuration and a process forproducing the same.

Another object of the present invention is to provide a process forproducing dendrite or asteroidal titanium dioxide micro-particles havinga length of 0.2 to 0.5 μm, a thickness of 0.05 to 0.1 μm, and a specificsurface area of 20 to 130 m² /gram comprising treating hydrated titaniumoxide particles with alkali, mixing instantaneously the resultingreaction product with an amount of hydrochloric acid in a ratio of 1 to4 mols of hydrochloric acid to one mol of the titanium oxide in saidreaction product to effect a reaction, then aging under heat at 85° C.or higher, and then drying or, if necessary, firing after the drying.

Still another object of the present invention is to provide dendrite orasteroidal titanium dioxide micro-particles, which are further improvedin dispersibility in media as well as in resistance to light, having alayer of at least one selected from oxides and hydrated oxides ofelements selected from the group consisting of aluminum, silicon,titanium, zirconium, tin and antimony coated on the surfaces of theparticles.

Still another object of the present invention is to provide a processfor producing dendrite or asteroidal titanium dioxide micro-particles,which are further improved in dispersibility in media as well as inresistance to light, comprising forming a slurry of the dendrite orasteroidal titanium dioxide micro-particles, adding to the slurry atleast one selected from the group consisting water soluble salts of eachelement selected from aluminum, silicon, titanium, zirconium, tin andantimony, neutralizing the resultant slurry to coat the surfaces of saidtitanium dioxide micro-particles with at least one selected from oxidesand hydrated oxides of said elements.

Still another object of the present invention is to provideelectroconductive dendrite or asteroidal titanium dioxidemicro-particles comprising the dendrite or asteroidal titanium dioxidemicro-particles having an electroconductive layer of tin oxidecontaining antimony or indium oxide containing tin coated on thesurfaces of the particles.

Still another object of the present invention is to provide anelectroconductive paint, electroconductive resin composition, paint formagnetic recording media, electroconductive paint for magnetic recordingmedia, sunscreen cosmetic, UV screening paint, UV shielding plasticcomposition, and the like which comprise the dendrite or asteroidaltitanium dioxide micro-particles

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electron microphotograph (a magnification of 100,000)showing the grain structure of the dendrite or asteroidal titaniumdioxide micro-particles (Sample A) of the present invention produced inthe step (2) of Example 1.

FIG. 2 is an electron microphotograph (a magnification of 100,000)showing the grain structure of the dendrite or asteroidal titaniumdioxide micro-particles (Sample B) of the present invention produced inthe step (3) of Example 1.

FIG. 3 is an electron microphotograph (a magnification of 100,000)showing the grain structure of the rod type titanium dioxidemicro-particles (Sample C) produced in the step (2) of ComparativeExample 1.

FIG. 4 is an electron microphotograph (a magnification of 100,000)showing the grain structure of the rod type titanium dioxidemicro-particles (Sample D) produced in the step (3) of ComparativeExample 1.

DETAILED DESCRIPTION OF THE INVENTION

The present inventors have made an intensive research to overcome theaforementioned difficulties, and as a result, it has been found thattitanium dioxide micro-particles having a specific length, a specificthickness, a specific surface area and a specific configuration, whichcan be produced under specific conditions, have an excellent UVshielding property and are good in transparency and surface flatness inapplication systems, and that none of sintering and great deformation ofparticles are caused by firing to allow the particles to possess good UVshielding property, excellent resistance to light, superior transparencyand surface flatness in application systems, and that the titaniumdioxide micro-particle substrate may be subjected to anelectroconductivity imparting treatment to produce anelectroconductivity imparting agent having an extremely excellentperformance which can find a great many potential applications. Thepresent invention is accomplished based on the above findings.

That is, the present invention relates to (1) a process for producingdendrite or asteroidal titanium dioxide micro-particles comprisingtreating hydrated titanium oxide particles with an alkali, mixinginstantaneously the resulting reaction product with an amount ofhydrochloric acid in a ratio of 1 to 4 mols of hydrochloric acid to onemol of the titanium oxide in the reaction product to effect a reaction,then aging under heat at 85° C. or higher, and then drying or, ifnecessary, firing after drying, (2) a process for producing dendrite orasteroidal titanium dioxide micro-particles comprising forming a slurryof the dendrite or asteroidal titanium dioxide micro-particles obtainedby the above process (1), adding to the slurry at least one selectedfrom the group consisting water soluble salts of each element selectedfrom aluminum, silicon, titanium, zirconium, tin and antimony,neutralizing the resultant slurry to coat the surfaces of said titaniumdioxide micro-particles with at least one from oxides and hydratedoxides of said elements, (3) dendrite or asteroidal titanium dioxidemicro-particles having a length of 0.2 to 0.5 μm, a thickness of 0.05 to0.1 μm, and a specific surface area of 20 to 130 m² /gram, which areproduced by the above process (1), (4) products obtained by the aboveprocess (2), and (5) electroconductive paints, electroconductive resincomposition, magnetic recording medium paints, electroconductive paintsfor magnetic recording medium, sunscreen cosmetics, UV screening paintsand UV shielding plastic compositions, which are prepared using theaforementioned products.

The dendrite or asteroidal titanium dioxide micro-particles according tothe present invention have a dendrite or asteroidal configuration, whereacicular and/or rod type particles assemble or combine to form a bundleshape and then a plurality of bundle shapes combine radially to form asingle composite particle, which is completely different fromconventional acicular, rod-like or spherical as is apparent from Figures(electron microphotographs). As used here in connection with suchconfiguration, the term "length" of the dendrite or asteroidal titaniumdioxide micro-particles according to the present invention means thelongest dimension of each single composite particle, and the term"thickness" means the largest diameter in the direction of a short axisof the bundle shape composing the longest part. The dendrite orasteroidal titanium oxide particles of the present invention have alength of 0.2 to 0.5 μm, a thickness of 0.05 to 0.1 μm and a specificsurface area of 20 to 130 m² /gram. As described later, when thetitanium dioxide micro-particles are employed as substrate particles,subjected to an electroconductive coating treatment and then fired, thespecific surface area of the particulate titanium oxide substrate shouldbe in the range of 70 m² /gram to 130 m² /gram, preferably in the rangeof 70 m² /gram to 100 m² /gram, while when the fired titanium dioxidemicro-particles are employed as substrate particles, subjected to theelectroconductive coating treatment, and further fired, the specificsurface area of the particulate titanium oxide substrate should be inthe range of 20 m² /gram to less than 70 m² /gram, preferably 20 m²/gram to 50 m² /gram.

The dendrite or asteroidal titanium dioxide micro-particles of thepresent invention are useful for various sunscreen preparations, UVscreening paints, and UV shielding plastic compositions, and may befurther improved in dispersibility and resistance to light of thetitanium oxide particles in the dispersion thereof in medium bydepositing at least one from hydrated oxides of metals such as aluminum,silicon, titanium, zirconium, tin and antimony onto the surfaces of themicro-particles. The amount of the oxides or hydrated oxides of metalsto be deposited should be in the range from 1 to 100% by weightexpressed as a total of metal oxides based on the titanium dioxide.

The dendrite or asteroidal titanium dioxide micro-particles of thepresent invention which have been subjected to the electroconductivecoating treatment may be incorporated as electroconductivity impartingagent or substrates into plastics, rubbers, fibers and the like, toproduce usable electroconductive compositions such as electroconductiveplastics, electroconductive paints, magnetic paints, electroconductiverubbers, electroconductive fibers and the like. The electroconductivecoating treatment may be accomplished by forming an electroconductivelayer comprising tin oxide doped with antimony or indium oxide dopedwith tin on the surfaces of the dendrite or asteroidal titanium dioxidemicro-particles. The formation of the electroconductive layer may beperformed by, for example, adding a water soluble tin compound and awater soluble antimony compound to the titanium dioxide micro-particlesto deposit hydrated tin oxide and hydrated antimony oxide onto thesurfaces of the titanium dioxide micro-particles, and then firing toform the electroconductive layer comprising tin oxide doped withantimony. In this case, the amount of tin oxide should be in the rangefrom 10 to 150% by weight, preferably 30 to 100% by weight as SnO₂. Anamount of tin oxide lower than the indicated range may make it difficultto form continuous electroconductive layer with no desiredelectroconductivity being achieved. Addition of too much tin oxide isuneconomical because further improvement in electroconductivitycorresponding to an increase in the amount over the indicated range cannot be expected. The amount of the aforementioned antimony oxide in theelectroconductive layer should be in the range from 5 to 40% by weight,preferably 10 to 30% by weight as Sb₂ O₃ based on tin oxide (SnO₂). Anamount of antimony oxide lower than the stated range may result inimpossibility of attaining desired electroconductivity. Too high anamount of antimony oxide is undesirable because it may cause reducedelectroconductivity and intensive coloration with antimony oxide.

A process for producing the dendrite or asteroidal titanium dioxidemicro-particles according to the present invention is described under.

First, an alkali is added to an aqueous dispersion of hydrated titaniumoxide and then heated at a temperature of 90° to 100° C. to produce areaction product, the pH of which is adjusted to neutral to separatesolid from liquid, and then the solid is washed with water. The alkalisto be used in the treatment with alkali include sodium hydroxide andsodium carbonate, and the use of an aqueous solution of sodium hydroxideis preferred. The aforementioned washed reaction product is dispersed inwater to produce a dispersion. The dispersion and hydrochloric acid areinstantaneously mixed in a ratio of hydrochloric acid of 1 to 4 molsrelative to one mol of the titanium oxide in the dispersion to effect areaction. More practically speaking, for example, the hydrochloric acidis added to the dispersion at a speed of at least 2 mols/second withstirring in a ratio of hydrochloric acid of 1 to 4 mols to one mol ofthe titanium oxide of the reaction product in the dispersion.Alternatively, the dispersion and hydrochloric acid are placed at a timein a vessel in the ratio as described above and mixed with stirring. Ifdesired, the dispersion may be incorporated into the hydrochloric acid.Then the resulting dispersion is aged at a temperature of 85° to 100°C., preferably 90° to 100° C. for one hour or more to produce an aqueousdispersion of dendrite or asteroidal titanium dioxide micro-particleswhich is filtrated, washed and dried to produce powdery dendrite orasteroidal titanium dioxide micro-particles, and if necessary, thepowder may be fired at a temperature of 400° to 700° C. to produce apowder of dendrite or asteroidal titanium dioxide micro-particles.

The hydrated titanium oxides, a titanium source of the dendrite orasteroidal titanium dioxide micro-particles include those obtained fromthe hydrolysis or the hydrolysis under neutralization of a solution oftitanyl sulfate or a solution of titanium tetrachloride. Morepractically, for example, an aqueous solution of titanium tetrachlorideis neutralized with an aqueous solution of sodium hydroxide whilemaintaining at room temperature to precipitate colloidal amorphoustitanium hydroxide which is aged under heat to produce a fine rutiletitania sol for practical use.

Onto the surfaces of the dendrite or asteroidal titanium dioxidemicro-particles obtained as described above, there may be deposited orcoated at least one of oxides and hydrated oxides of metals such asaluminum, silicon, titanium, zirconium, tin and antimony. This coatingtreatment may be accomplished by, for example, dispersing the dendriteor asteroidal titanium dioxide micro-particles into water to produce aslurry, and if necessary, wet ground and classified, and thereafter atleast one selected from the group consisting of water soluble salts ofaluminum, silicon, titanium, zirconium, tin and antimony is added to theslurry in an amount of 1 to 100 expressed as a total amount of oxidesbased on the titanium dioxide, and then the slurry is neutralized withan acidic solution such as sulfuric acid, hydrochloric acid and the likewhen the addition of the water soluble salts renders the dispersionalkaline, or with a basic solution such as an aqueous alkaline solutionof sodium hydroxide, ammonia water and the like when the addition of thewater soluble salts renders the dispersion acidic, to effect the coatingdeposition onto the surfaces of the titanium dioxide micro-particleswhich then are separated, dried and ground. This coating treatmentallows improvement in dispersibility and durability of the dendrite orasteroidal titanium dioxide micro-particles in dispersion media.

As described above, the dendrite or asteroidal titanium dioxidemicro-particles of the present invention are useful for varioussunscreen cosmetics, UV screening paints, and UV shielding plasticcompositions, and the electroconductive dendrite or asteroidal titaniumdioxide micro-particles which have been subjected to theelectroconductive imparting treatment may be incorporated aselectroconductivity imparting agent or substrates into plastics,rubbers, fibers and the like, to produce usable electroconductivecompositions such as electroconductive plastics, electroconductivepaints, magnetic paints, electroconductive rubbers, electroconductivefibers and the like.

The dendrite or asteroidal titanium dioxide micro-particles of thepresent invention can be used in sunscreen cosmetics in any one ofvarious forms such as lotion, cream, paste, stick and emulsion, intowhich additives such as oily components, moisturizing agents,surfactants, flavors, preservatives, water, alcohols and thickners maybe incorporated.

The dendrite or asteroidal titanium dioxide micro-particles can be usedin production of UV screening plastics or electroconductive plastics byincorporating into synthetic resins such as vinyl chloride resins, ABSresins, polyethylenes, polypropylenes, vinylidene chloride,polystyrenes, polycarbonates, nylon, EVA resins, polyacetal resins,polyamide resins, phenolic resins, melamine resins, acrylic resins,polyester resins, urea resins, silicone resins and fluorinated resins.

When the dendrite or asteroidal titanium dioxide micro-particles can beused in UV screening paints, electroconductive paints or magneticpaints, they are incorporated into, for example, polyvinyl alcoholresins, vinyl chloride-vinyl acetate resins, acrylic resins, epoxideresins, urethane resins, alkyd resins, polyester resins, ethylene vinylacetate copolymers, acrylic styrene copolymers, cellulose resins,phenolic resins and amino resins, by dispersing in water or solvents,With electroconductive paints, they may be coated on an insulatingsubstrate such as a sheet of paper and polymer films to form a lightelectroconductive coating having an excellent adherence on the surfacesthereof which can be used as electrostatic recording paper,electrophotographic copying paper and antistatic films.

The paints to be used in production of magnetic recording media areuseful for improving adhesion strength between a non-magnetic supportand a magnetic layer, antistatic property of magnetic recording media,film strength, thinner magnetic layer, dispersibility of non-magneticunderlayer accompanied with surface flattening, and surface flatness.Among others, a recent tendency of increasing the recording density inmagnetic recording requires remarkably shorter and shorter wavelengthsfor recording, which is accompanied with the requirement of making themagnetic layer of magnetic recording media thinner. However, thereducing of the thickness of the magnetic layer tends to causemanifestation of influence of the support onto the surfaces of themagnetic layer resulting inevitably in deterioration in electromagneticproperties. For this reason, for example, an attempt has been made toavoid the influence of the surface roughness of the support by providinga non-magnetic undercoat layer on the surface of the non-magneticsupport and then a magnetic overcoat layer on the undercoat layer,simultaneously making the magnetic layer thinner to increase the output.In this connection, the dendrite or asteroidal titanium dioxidemicro-particles of the present invention may be filled in thenon-magnetic undercoat layer to render the surface flatness of themagnetic overcoat layer more preferable. The proportion of the titaniumdioxide micro-particles to be filled in the non-magnetic undercoat layershould be on the order of 20 to 80% by volume.

The dendrite or asteroidal titanium dioxide micro-particles of thepresent invention can be used in production of electroconductiverubbers, for example, by incorporating into known conventionalelastomers such as silicone rubbers, isoprene rubbers, styrene-butadienerubbers, butadiene rubbers, butyl rubbers, butadiene-acrylonitrilerubbers, ethylene-propylene-diene polymers, ethylene-propylene rubbers,fluorinated rubbers, ethylene-vinyl acetate copolymers, chlorinatedpolyethylenes, acrylic rubbers, chloroprene rubbers, urethane rubbers,polysulfide rubbers, chlorosulfonated polyethylene rubbers, andepichlorohydrin rubbers.

The dendrite or asteroidal titanium dioxide micro-particles of thepresent invention can be used in production of electroconductive fibers,for example, by incorporating into spinnable resins such as polyamideresins, polyester resins, polyolefin resins, polyvinyl resins andpolyether resins.

The thus obtained electroconductive compositions are advantageous incost because less amount thereof to be incorporated into resin bindersis required to achieve a high electroconductivity than the conventionalelectroconductive compositions formulated with sphericalelectroconductive particles. Since such a small amount of thecompositions to be incorporated is required, they can be utilizedwithout causing any reduction in strength of the binders.Electroconductive paints having a high concentration of the compositionsenable attainment of desired electroconductivity with a thinner coatingfilm.

The dendrite or asteroidal titanium dioxide micro-particles of thepresent invention can be used in various cosmetics and paints where theymay be coated with at least one of organic agents for treatments to beused in the fields of preparations and paints such as carboxylic acids,polyhydrics, amines, siloxanes and silane coupling agents, whereby theymay be improved in the dispersibility into cosmetics and paints as wellas the durability of coating films.

EXAMPLE 1

(1) Hydrated titanium oxide particles obtained by hydrolysis of anaqueous solution of titanium tetrachloride were dispersed in aconcentration of 100 grams/liter expressed as TiO₂ to produce an aqueousdispersion. To 2 liters of this dispersion there were added 1400 gramsof an aqueous 48% sodium hydroxide solution with stirring, the mixturewas heated at 95° C. for 120 minutes, filtered and sufficiently washed.The washed cake was repulped in water to produce an aqueous dispersionhaving a concentration of 100 grams/liter expressed as TiO₂. 1.5 litersof this aqueous dispersion were placed in a flask equipped with a refluxcondenser and 570 grams of a 35% hydrochloric acid were instantaneouslyadded with stirring at a speed of 4 mols/second, and thereafter themixture was aged under heat at 95° C. for 120 minutes to produce anaqueous dispersion containing dendrite or asteroidal titanium dioxidemicro-particles.

(2) The aqueous dispersion obtaind in the above step (1) was filtered,washed, and the resultant washed cake was dried at 120° C. for one wholeday and night to produce dendrite or asteroidal titanium dioxidemicro-particles having a rutile crystal length of 0.30 μm, a thicknessof 0.055 μm, a specific surface area of 79 m² /gram (Sample A).

(3) The aqueous dispersion containing dendrite or asteroidal titaniumdioxide micro-particles obtaind in the above step (1) was filtered,washed, and the resultant washed cake was dried at 120° C. for one wholeday and night, followed by firing in an electric furnace at 500° C. forone hour to produce dendrite or asteroidal titanium dioxidemicro-particles having a length of 0.27 μm, a thickness of 0.05 μm, aspecific surface area of 28 m² /gram (Sample B) .

COMPARATIVE EXAMPLE 1

(1) Hydrated titanium oxide particles obtained by hydrolysis of anaqueous solution of titanium tetrachloride were dispersed in aconcentration of 100 grams/liter expressed as TiO₂ to produce an aqueousdispersion. To 2 liters of this dispersion there were added 1400 gramsof an aqueous 48% sodium hydroxide solution with stirring, the mixturewas heated at 95° C. for 120 minutes, then filtered and sufficientlywashed. The washed cake was repulped in water to produce an aqueousdispersion having a concentration of 100 grams/liter expressed as TiO₂.1.5 liters of this aqueous dispersion were placed in a flask equippedwith a reflux condenser and 570 grams of a 35% hydrochloric acid wereadded over 30 minutes with stirring, and thereafter the mixture washeated to 95° C. and aged for 90 minutes to produce an aqueousdispersion containing rod type titanium dioxide micro-particles.

(2) The aqueous dispersion obtained in the above step (1) was filtered,washed, and the resultant washed cake was dried at 120° C. for one wholeday and night to produce rod type titanium dioxide micro-particleshaving a rutile crystal length of 0.07 μm, an aspect ratio of 7, aspecific surface area of 99 m² /gram (Sample C).

(3) The aqueous dispersion containing rod type titanium dioxidemicro-particles obtained in the above step (1) was filtered, washed, andthe resultant washed cake was dried at 120° C. for one whole day andnight, followed by firing in an electric furnace at 500° C. for one hourto produce rod type titanium dioxide micro-particles having a length of0.07 μm, a thickness of 0.035 μm, a specific surface area of 39 m² /gram(Sample D).

EXAMPLE 2

The aqueous dispersion containing dendrite or asteroidal titaniumdioxide micro-particles obtained in the above step (1) of Example 1 washeated to 90° C., and a solution of 150 grams of tin chloride (SnCl₄.5H₂O) and 25 grams of antimony chloride (SbCl₃) dissolved in 200milliliters of an aqueous 6N hydrochloric acid solution was added to theheated dispersion together with an aqueous 10% sodium hydroxide solutionover 60 minutes so as to keep the pH of the dispersion at 2 to 3,thereby depositing hydrates of tin oxide and antimony oxide onto thesurfaces of the dendrite or asteroidal titanium dioxide micro-particles.In this case, the final pH of the dispersion was 3. Then the aqueousdispersion of the coated dendrite or asteroidal titanium dioxidemicro-particles was filtered, washed until the conductivity of thefiltrate reached less than 50 μS, the cake of the coated dendrite orasteroidal titanium dioxide micro-particles was dried at 120° C. for awhole day and night and then fired in an electric furnace at 500° C. forone hour to produce electroconductive dendrite or asteroidal titaniumdioxide micro-particles having an electroconductive layer of tin oxidedoped with antimony, which comprised by weight 51.6% SnO₂ and 13.3% Sb₂O₃ based on the weight of TiO₂, coated on the surfaces and having alength of 0.36 μm, a thickness of 0.065 μm and a specific surface areaof 33 m² /gram (Sample E). Its powder resistance was determined to be 13Ω cm.

The dendrite or asteroidal titanium dioxide micro-particles obtained inthe above step (3) of Example 1 were dispersed in water to produce anaqueous dispersion having a concentration of 100 grams/liter expressedas TiO₂, and one liter of the dispersion was wet ground, heated to 90°C., and a solution of 100 grams of tin chloride (SnCl₄.5H₂ O) and 17grams of antimony chloride (SbCl₃) dissolved in 200 milliliters of anaqueous 6N hydrochloric acid solution was added to the heated dispersiontogether with an aqueous 10% sodium hydroxide solution over 60 minutesso as to keep the pH of the dispersion at 2 to 3, thereby depositinghydrates of tin oxide and antimony oxide onto the surfaces of thedendrite or asteroidal titanium dioxide micro-particles. In this case,the final pH of the dispersion was 3. Then the aqueous dispersion of thecoated dendrite or asteroidal titanium dioxide micro-particles wasfiltered, washed until the conductivity of the filtrate reached lessthan 50 μS, the cake of the coated dendrite or asteroidal titaniumdioxide micro-particles was dried at 120° C. for a whole day and nightand then fired in an electric furnace at 500° C. for one hour to produceelectroconductive dendrite or asteroidal titanium dioxidemicro-particles having an electroconductive layer of tin oxidecontaining antimony, which comprised by weight 51.7% SnO₂ and 13.4% Sb₂O₃ based on the weight of TiO₂, coated on the surfaces and having alength of 0.29 μm, a thickness of 0.05 μm and a specific surface area of33 m² /gram (Sample F). Its powder resistance was determined to be 8.0 Ωcm.

Its powder resistance was determined by first forming a powdery sampleunder a pressure of 100 Kg/cm² to a cylindrical pressed body ofdimensions of 18 mm in diameter×3 mm in thickness, and then measuringthe DC resistivity of the body according to the following formula:##EQU1##

COMPARATIVE EXAMPLE 2

The aqueous dispersion containing rod type titanium dioxidemicro-particles obtained in the above step (1) of Comparative Example 1was heated to 90° C., and a solution of 150 grams of tin chloride(SnCl₄.5H₂ O) and 25 grams of antimony chloride (SbCl₃) dissolved in 200milliliters of an aqueous 6N hydrochloric acid solution was added to theheated dispersion together with an aqueous 10% sodium hydroxide solutionover 60 minutes so as to keep the pH of the dispersion at 2 to 3,thereby depositing hydrates of tin oxide and antimony oxide onto thesurfaces of the rod type titanium dioxide micro-particles. In this case,the final pH of the dispersion was 3. Then the aqueous dispersion of thecoated rod type titanium dioxide micro-particles was filtered, washeduntil the conductivity of the filtrate reached less than 50 μS, the cakeof the coated rod type titanium dioxide micro-particles was dried at120° C. for a whole day and night and then fired in an electric furnaceat 500° C. for one hour to produce electroconductive rod type titaniumdioxide micro-particles having an electroconductive layer of tin oxidecontaining antimony, which comprised by weight 47.2% SnO₂ and 11.6% Sb₂O₃ based on the weight of TiO₂, coated on the surfaces and having alength of 0.1 μm, a thickness of 0.015 μm and a specific surface area of41 m² /gram (Sample G). Its powder resistance was determined to be 15 Ωcm.

The rod type titanium dioxide micro-particles obtained in the above step(3) of Comparative Example 1 were dispersed in water to produce anaqueous dispersion having a concentration of 100 grams/liter expressedas TiO₂, and one liter of the dispersion was wet ground, heated to 90°C., and a solution of 100 grams of tin chloride (SnCl₄.5H₂ O) and 17grams of antimony chloride (SbCl₃) dissolved in 200 milliliters of anaqueous 6N hydrochloric acid solution was added to the heated dispersiontogether with an aqueous 10% sodium hydroxide solution over 60 minutesso as to keep the pH of the dispersion at 2 to 3, thereby depositinghydrates of tin oxide and antimony oxide onto the surfaces of the rodtype titanium dioxide micro-particles. In this case, the final pH of thedispersion was 3. Then the aqueous dispersion of the coated rod typetitanium dioxide micro-particles was filtered, washed until theconductivity of the filtrate reached less than 50 μS, the cake of thecoated rod type titanium dioxide micro-particles was dried at 120° C.for a whole day and night and then fired in an electric furnace at 500°C. for one hour to produce electroconductive rod type titanium dioxidemicro-particles having an electroconductive layer of tin oxidecontaining antimony, which comprised by weight 51.0% SnO₂ and 13.1% Sb₂O₃ based on the weight of TiO₂, coated on the surfaces and having alength of 0.07 μm, a thickness of 0.04 μm and a specific surface area of29 m² /gram (Sample H). Its powder resistance was determined to be 12 Ωcm.

EXEMPLARY TEST 1

The titanium dioxide micro-particles of Samples A and C obtained inExample 1 and Comparative Example 1 were each incorporated in thefollowing formulation to produce a sunscreen cream:

    ______________________________________                                                             parts by weight                                          ______________________________________                                        (1)    Stearic acid        2.5                                                (2)    Bleached bees wax   3.5                                                (3)    Cetanol             3.5                                                (4)    Squalene            17.0                                               (5)    Glycerin monostearate                                                                             3.0                                                (6)    Titanium dioxide micro-particles                                                                  3.0                                                (7)    Methylparaben       0.1                                                (8)    Glycerin            12.0                                               (9)    Triethanolamine     1.0                                                (10)   Distilled water     54.1                                               (11)   Flavor              0.3                                                ______________________________________                                    

The components (1) to (6) were mixed under heat at 80° C. and added to amixture of components (7) to (10) mixed under heat at 80° C., and thewhole was intimately mixed by a homogenizing mixer and vigorouslystirred. The flavor (11) was added at around 45° C. to prepare asunscreen cream.

EVALUATION

Each of the creams was coated on a quartz glass sheet to a filmthickness of 25 μm and evaluated for transmittance with aspectrophtometer at a wave length of 750 to 300 nm. The results of theevaluation are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                  Transmittance (%)                                                               Visible                                                                  Sam- light range                                                                              UV A range UV B range                                         ple  (550 nm)   (375 nm)   (300 nm)                                    ______________________________________                                        Example 1                                                                              A      61.6       16.7      7.8                                      Comp. Ex. 1                                                                            C      85.9       56.7     41.5                                      ______________________________________                                    

EXEMPLARY TEST 2

(1) 20 grams of each of the Samples E, F, G and H obtained in Example 2and Comparative Example 2 were added to a glass bottle containing 30.6grams of acrylic resin (Acrydic A-165-45; 45% by weight solids,available from Dainihon Ink Chemical Industry Co.), 16.4 grams of aliquid mixture of toluene/butanol (1/1) and 50 grams of glass beads andthe whole was shaked for 30 minutes with a paint shaker to produce adispersion to be used as a mill base.

(2) To each mill base was added the acrylic resin and the liquid mixtureof toluene/butanol identical to those described above to attain aconcentration of each pigment of 20%, 30%, and 40%, to prepare a paint.The paint was applied on a sheet of art paper to a film thickness of 37μm on dryness and dried for 40 hours to produce a specimen. The artpaper specimens were evaluated for electric resistance by a digital ohmmeter (Model R-506 available from Kawaguchi Electric Manufacturing) andthe surface resistivity was calculated according to the followingformula: ##EQU2##

                  TABLE 2                                                         ______________________________________                                        Surface resistivity of the film applied                                       on the art specimen [Ω/□]                                                 Concentration of pigment in the film                                          [% by weight]                                                            Sample 20        30        40                                         ______________________________________                                        Example 2 E        2.3 × 10.sup.7                                                                    4.6 × 10.sup.6                                                                  1.0 × 10.sup.6                               F        1.0 × 10.sup.7                                                                    1.3 × 10.sup.6                                                                  3.2 × 10.sup.5                     Comp. Ex. 2                                                                             G        5.2 × 10.sup.7                                                                    8.0 × 10.sup.6                                                                  2.3 × 10.sup.6                               H        3.6 × 10.sup.7                                                                    3.2 × 10.sup.6                                                                  7.0 × 10.sup.5                     ______________________________________                                    

EXEMPLARY TEST 3

The titanium dioxide micro-particles of Samples B and D obtained inExample 1 and Comparative Example 1 were evaluated for photocatalyticactivity according to the following procedure:

0.2 gram of a sample and 10 cm³ of tetralin were placed in a flat bottompan type reaction vessel (50 cm³) which then was disposed on a tetralintest apparatus equipped with a water bath, magnetic stirrer, mercurymanometer and mercury lamp. The temperature of the water in the bath waskept at 40° C. while stirring with the magnetic stirrer, the valve ofthe mercury manometer was closed, and the reaction vessel was exposed tothe light rays from a 75 W mercury lamp at an illuminance of 2500 luxfrom above the vessel. During the irradiation, the difference inpressure in the mercury manometer was read every 5 minutes. The resultsof the evaluation are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                                   Irradiation time (min.)                                                   Sample                                                                              5     10     15  20   25  30   35  40                            ______________________________________                                        Example 1                                                                              B       5     10   15  19   25  29   34  39                          Comp. Ex. 1                                                                            D       6     13   20  25   32  39   46  52                          Control  --      2      5    8  10   12  15   17  20                          ______________________________________                                         Note: A larger numeral means a higher photocatalytic activity.           

The present invention lies in dendrite or asteroidal titanium dioxidemicro-particles which can be produced by a simple process comprisingtreating an aqueous dispersion of hydrated titanium oxide with alkali,then adding rapidly hydrochloric acid to the dispersion which is agedunder heat with or without further firing after the aging. On thesurfaces of the titanium dioxide micro-particles, various metalcompounds may be coated or deposited, whereby the titanium dioxidemicro-particles can be rendered useful in UV screening or shieldingpreparations or paints or as substrate materials capable of impartingelectroconductivity. Thus the present invention has a great effect inindustry.

What is claimed is:
 1. Dendritic or asteroidal titanium dioxidemicro-particles having a length of 0.2 to 0.5 μm along a longestdimension of each micro-particle, a thickness of 0.05 to 0.1 μm as thelargest diameter in the direction of a short axis of the micro-particle,and a specific surface area of 20 m² /gram to 130 m² /gram.
 2. Dendriteor asteroidal titanium dioxide micro-particles according to claim 1,wherein said particles have a specific surface area of 70 m² /gram to130 m² /gram.
 3. Dendrite or asteroidal titanium dioxide micro-particlesaccording to claim 1, wherein said particles have a specific surfacearea of 20 m² /gram to less than 70 m² /gram.
 4. Dendrite or asteroidaltitanium dioxide micro-particles comprising the dendrite or asteroidaltitanium dioxide micro-particles according to claim 1 having a layer ofat least one selected from oxides and hydrated oxides of elementsselected from the group consisting of aluminum, silicon, titanium,zirconium, tin and antimony coated on the surfaces of the particles. 5.Dendrite or asteroidal titanium dioxide micro-particles comprising thedendrite or asteroidal titanium dioxide micro-particles according toclaim 2 having a layer of at least one selected from oxides and hydratedoxides of elements selected from the group consisting of aluminum,silicon, titanium, zirconium, tin and antimony coated on the surfaces ofthe particles.
 6. Dendrite or asteroidal titanium dioxidemicro-particles comprising the dendrite or asteroidal titanium dioxidemicro-particles according to claim 3 having a layer of at least oneselected from oxides and hydrated oxides of elements selected from thegroup consisting of aluminum, silicon, titanium, zirconium, tin andantimony coated on the surfaces of the particles.
 7. Electroconductivedendrite or asteroidal titanium dioxide micro-particles comprising thedendrite or asteroidal titanium dioxide micro-particles according toclaim 1 having an electroconductive layer of tin oxide containingantimony or indium oxide containing tin coated on the surfaces of theparticles.
 8. Electroconductive dendrite or asteroidal titanium dioxidemicro-particles comprising the dendrite or asteroidal titanium dioxidemicro-particles according to claim 2 having an electroconductive layerof tin oxide containing antimony or indium containing tin coated on thesurfaces of the particles.
 9. Electroconductive dendrite or asteroidaltitanium dioxide micro-particles comprising the dendrite or asteroidaltitanium dioxide micro-particles according to claim 3 having anelectroconductive layer of tin oxide containing antimony or indiumcontaining tin coated on the surfaces of the particles.